Patent Publication 1 discloses an invention relating to a fuel battery containing a layered metal oxide NaCo2O4 as a solid electrolyte. The object of this invention is to obtain strong electromotive force even at about 20 to 80° C., i.e., such low temperatures as from about room temperature, without using deleterious substances or platinum.
Patent Publication 1 teaches that a solid electrolyte layer of less than 0.3 mm thick tends to be insufficient in strength. In the Examples of this publication, samples having a solid electrolyte layer formed in the thickness of about 1 mm were used to fabricate fuel batteries, and their electromotive force was evaluated. As can be seen from the current-voltage curve shown in FIG. 6 of the publication, the open circuit voltage (OCV) at the operation temperature of 75° C. was about 0.68V, and the current density was measured only up to 30 mA/cm2.
Practical use of such fuel batteries with the solid electrolyte layer requires reduced internal resistance of the solid electrolyte layer by making it thinner than the conventional ones. On the other hand, in order to bring out full performance of fuel batteries, the solid electrolyte layer needs to be dense.